Problem: Frances can complete $91$ oil changes in $7$ days. How many oil changes can Frances complete in $11$ days?
Explanation: We're trying to find the number of oil changes Frances can complete in $11$ days. If in ${7}$ days, Frances can complete ${91}$ oil changes, then in $1$ day, she can complete $13$ oil changes.  So, in ${11}$ days, Frances can complete ${143}$ oil changes.